With respect to the measurement of temperature, the world is at present governed by the IPTS-68 (International Practical Temperature Scale of 1968) scale. This scale defines certain "fixed points", such as the triple point of water (0.01 degree C.), the boiling point of water (100 degrees C.), etc., and then specifies a means of interpolating between these fixed points by using an SPRT (Standard Platinum Resistance Thermometer) together with a set of defining equations which allow the relationship of SPRT resistance versus SPRT temperature to be used to derive all other temperatures which are in between the fixed points.
Prior to the IPTS-68, the IPTS-48 (1948) was used to define temperature worldwide. On Jan. 1, 1990, a new scale, known tentatively as the ITS-90 (International Temperature Scale of 1990) is scheduled to supersede the IPTS-68. In the USA, all IPTS-68 calibrations are traceable to the National Institute of Standards and Technology (NIST), formerly known as the National Bureau of Standards (NBS).
At present, precision temperature measurements are made in a very cumbersome way.
First, a temperature probe is calibrated against the IPTS-68, either directly (primary calibration) by using fixed points and defining equations, or indirectly (secondary calibration) by comparing it against a previously calibrated SPRT. The temperature probe under calibration is then characterized with regard to its resistance value and change of that value versus applied temperature. This information is recorded on a Calibration Certificate. The probe is now said to have been calibrated.
Second, the user now obtains a precision resistance measuring device such as an ohmmeter which has its own Calibration Certificate and is quite expensive. The probe is now connected to the resistance measuring device and the value of resistance is recorded. This is done by a skilled technician and takes considerable time.
Third, the recorded value of resistance is now converted to the appropriate temperature scale (IPTS-48, IPTS-68 or ITS-90), and units (degrees C, F, K) which is tedious and error prone. Oftentimes this is still done by hand, using "look-up tables" and manual interpolations and calculations.
Even with present state-of-the-art equipment where the resistance measuring device has the required intelligence to read the probe resistance directly in temperature, a minimum of two error and uncertainty sources exist, the probe calibration uncertainty and the uncertainty of the resistance measuring device. Needless to say, these devices are bulky, difficult to operate and cost up to ten times as much as the probe itself.